Hum Free Splitter Problem

[Unlikekurt]

Hello all.
I built a hum free aby via RG's schematics.  I also used the charge pump.

I tried it out and the following is what occurred.

One guitar.  Two amplifiers.

Now, this is the part that i think i need to mention.
One of the amp's is an one with only a two prong AC plug.

When I plug that amp in, depending on the orientation of the blades in the socket i get a great deal of hum from both amplifiers.
If I flip the plug around, all the hum goes away.

Also, when the plug is oriented so that there is hum, if i unplug that amp's input cable from the ABY and touch the sleeve with my hand and then touch the chassis ground of the ABY i can feel current flowing through my hands.  *I also tried this same procedure with a completely passive ABY boy that i have sitting around and the same thing occurred.

Both of my outputs are jack shield's are connected to their respective transformers only.  Neither is referenced to the circuit/chassis ground.

I tried to connect a wire from one of the output jacks to ground and found that it did indeed reduce the amount of hum.  However, if i, instead of referencing it to the chassis ground, attach it to the shield lug of the other output jack, all of the hum goes away instantly.

So, basically, is there something up with my hum free aby or are these things due to the old two pronged guitar amp?

Thanks for any help.  I'm killing myself trying to figure out if this build is working or not?!

James

[R.G.]

One of the amp's is an one with only a two prong AC plug.

When I plug that amp in, depending on the orientation of the blades in the socket i get a great deal of hum from both amplifiers.
If I flip the plug around, all the hum goes away.
...
So, basically, is there something up with my hum free aby or are these things due to the old two pronged guitar amp?

Two-prong power cords have some things wrong with them in today's world. You've found one of them. The interior "death cap" is intended to attach the chassis to one or the other of the AC power lines. It also causes leakage of AC. Flipping the plug over changes which side the death cap is leaking to, and that varies the hum. A lot.

Just out of curiousity, does the two-prong amp hum when it's flipped the "hummy" way if you disconnect the input cord?

I would never use a two-prong amp for other than a quick test. It's too dangerous. And three-prong amps are quieter, if the conversion is done correctly.

So my advice is to convert the two-prong amp first. A leaky two-prong can defeat a lot of hum prevention measures.

[Unlikekurt]

thanks for the response RG.  I had a feeling it had something to do with the AC polarity.
If the amp is plugged in to the socket in the orientation that produces the most hum and i remove the input cord from the amplifier it doesn't really hum that much by itself.
However, if i then touch the volume knob it does hum a bunch more than when the plug is orientated the other way around.

In this scenario the hum is being caused by AC leakage yes?
If you have the time, would it be possible for you to explain what is happening inside the pedal?  I would like to understand how the noise is getting to the second amp.

Also, one last question.  If the hum in this scenario is in fact due to the reversed AC polarity between the two amplifiers, how would i go about testing the aby pedal to ensure that it is in fact properly working?  ie: how do i make the ground loop so i can see if the pedal puts the ground hum to an end?

thanks again RG!!

james

[Tony Forestiere]

I am probably overstepping my boundary, but I would try the "Divide and Conquer" method. Remove the two-pronged amp from the equation by borrowing a three-pronged amp (or battery powered practice amp), and see if you hear a difference.
My stupid guess. 

[R.G.]

I am probably overstepping my boundary, but I would try the "Divide and Conquer" method. Remove the two-pronged amp from the equation by borrowing a three-pronged amp (or battery powered practice amp), and see if you hear a difference.
My stupid guess. 

Oh, sure, suggest the simple, direct, most useful thing to do!   


My forehead is getting flatter as I go. 

[Tony Forestiere]

Thanks.  Street smarts.

[Unlikekurt]

Tony
of course that would work 

I guess what i'm trying to ask is, how to create the problem to see if the humfree aby works correctly?

If i plug in two known amplifiers and then plug my guitar into the humfree and don't hear any hum, does that mean the box is working or in that particular scenario perhaps there wasn't going to be any hum to begin with?

thanks
james

[R.G.]

One of the fundamentals of debugging is to change only one thing, and see if that affects the problem. If it does, something about what you changed is involved in the problem. It may not be what you think is involved in the problem, but it bears on the issue.

You're right in that a different three-wire amp may be quiet - but the two-wire amp was quiet in one position. It doesn't tell you that the splitter is working right. Two-wire amps all by themselves hum if the plug is flipped the wrong way, or if the "line reverse" switch is in the wrong position. That's why there's a line reverse switch. The hum in this t hing is from internal AC connection issues. It is possible that some two-wire amps generate so much AC leakage that even a signal isolator may not cure it.

You haven't said, but I suspect that you may have wanted an isolating splitter so you could use the two-wire amp without hum and without modifying it. That's a plausible set of issues. However, you're still left with the isolator may not be curing the as-yet unknown issue with amp hum. Or it may be and the amp(s) generate so much hum in one condition that it can't cope.

Talking this out makes me feel a bit less dopey about my first advice. The elves in the back of my head leapt right to a side issue to work with instead of the question you asked. I *think* the elves did this because there are so many ways that hum gets into a multi-amp setup that eliminating a known hum issue (i.e. the two-wire cord) is a good thing to do to nibble off a piece of the problem. Granted, temporarily subbing another amp is a more expedient test; I think the elves were saying that the two-wire had to be dealt with eventually anyway. I think.

A lot may hinge on what kind of setup the two-wire amp is. What is it? Condition? Age?

[wavley]

Last night I was playing around with my RG inspired splitter and putting an amp I don't often use (because I haven't gotten around to converting it to 3 prong yet, it's an old Oliver) and had your same problem.  The reason I use the splitter is to keep my pedalboard from ground looping with my amps (they all run off the same power strip and are grounded), generally I don't have a problem with any of my old amps that I've converted to newer power (plus that unused polarity switch hole makes a great place to put a bias pot) 

Anyways, same thing hum really bad plugged in one way, less the other... clip lead chassis to chassis (of a properly grounded amp) no more hum.  Time to convert to grounded power!  Honestly, I'm not sure why I haven't gotten around to it, it's quite likely that it's the only thing in my whole studio that isn't properly grounded.

[Unlikekurt]

RG: i'm laughing about those elves!!
Actually, the 2 prong old amp isn't a part of the common or intended setup at all.  I've only got one cabinet in the apartment at this moment so i only had one "real" amp to utilize.  the two pronger was an old ampeg buster (identical to pignose aside from badge and knob).  
The splitter was actually made as a commission.  The player was having hum issues when using a fender bassman and a ampeg v4 simultaneously.
I believe the setup that was being utilized was a passive arrangement with parallel outputs.  Then a volume pedal in front of the bassman to take it in and out.
yes yes yes i know i know, why don't you go on and try it out with the bassman and the v4?  
the player is no longer using the setup and i no longer have access to the bassman!

side note: i utilized your 2005 improved hum-free design as the basis for my project and incorporated a charge pump.  i used one 3PDT to control the on/off of each channel.  When the channel is disengaged, the first pole shunts the junction of the two 10K resisters before the output buffer IC to ground (this ground is supplied by the Y switch only when Y mode is disengaged; i believe your new design employs this methodology as well).  The second pole lifts the signal from ever reaching the first of the two R10K's.  the third is for the individual channel LED.  So in essence, my switching scheme is very much akin to your new 2012 design (i happened upon it yesterday, without the second TL072 acting as buffer's for each "channel" is signal loss noticeable when "Y" mode is engaged?  I've also wondered why the first TL072 when each output is individually buffered?) but i'm using both TL072's and the old shunting arrangement.

Thanks again!

Wavley: for safety alone it makes sense to only utilize 3 prong cords.  maybe i'll get around to those little ampeg amps at some point!

[Unlikekurt]

So I got my hum free working just fine.  WITH A BATTERY
when i use the power adapter everything gets a buzz to it.  I've checked the adapter and it is - center + barrel as the pedal is wired and it is providing 9vdc.
It is this model (http://www.cedist.com/products/M-PAS-S)
Could this be the particular model of adapter or adapters in general?  If the later what type of solution could i try?  Is this an instance where some sort of RF blocking measure would be of use?  Perhaps an RC network from output A to input ground?  RG i'm hoping you're going to read this

thanks
James

[R.G.]

Using a power adapter adds yet a third AC line and ground connection. In an ideal world, the built-in isolation of the adapter's internal transformer would prevent this from being an issue, but there is always stray capacitance from everything to everything else, and a little always gets through. Exactly how much is "a little" is something that each manufacturer fights all the time.

The real underlying issue is that we live in a world saturated and permeated with power line electrical and magnetic fields. It comes from nearly everything we use. Then we go use extremely high gain amplifiers with very high input impedances to amplify our guitars, and expect that none of the hum will get in.    

There are many ways hum leaks in. Some of it is honest leakage by capacitance. Some of it is loops of wire - like, unfortunately, our ground shields/wires - picking up the magnetic part of the power line radiation. Some of it is leakage through a transformer, some is leakage AROUND a transformer through a dust layer. Some is a difference in "ground" between the neutral side of a power line on one outlet and another outlet caused by the power current flowing through the wires and causing a current times voltage drop at the power line frequency. Some of it is the buildings we live and work in not being wired "right" - I use quotations because "right" can only be an approximation for signal purposes. Some of it is squarks of RF generated by the zillions of rectifiers slamming off at power line frequency as they rectify the power line current. Some of it is from ...

Well, you see where this is going. Avoiding hum never means "getting zero hum". You can't do it. Avoiding hum means suppressing the top ten - or twenty, or fifty - causes until we get below the threshold where our brains ignore it as being not there. Like rust, power-line hum never sleeps. I don't know about the CE Distribution unit - they say "Power-All"; is that the old Godlyke unit? - as being especially good or bad. Could be a problem, could be just fine. Could be something about this setup where the internal power rectification in the power adapter is transmitting blips of RF at hum frequencies in spite of that ferrite filter on the output cord.

The splitter attacks a number of hum issues that arise when you use two different amplifiers. The two amplifiers are attached to the AC power line at two different places, and each leaks a bit different amount of AC power line into its chassis. The situation is worst if one or both of them are two-wire units, but even two three-wire units can, under certain circumstances, cause hum in a multi-amplifier setup. The splitter, or anything with coils of wire in it, can even cause hum itself if you put it right over a magnetic hum source, like a power transformer.

It's like the saying, life is a journey, not a destination. I don't have The Answer on hum, other than you have to work with the situation you have, whatever that is.

Good. I feel better, having pontificated for a while. 

I notice that you say "when use the power adapter everything gets a buzz to it".  Hum comes in several flavors, and it takes some thinking and experience to separate them out by ear. Real, no-fooling power line hum is 60Hz (or 50Hz, depending on the country). That's a musical note below the low E on a guitar. Ripple from full-wave rectification is twice the power line frequency, an octave up. Buzzing hum happens when something happens at either power line frequency or twice power line frequency. Your ear fills in the power line frequency that isn't really there. Fluorescent lights make buzzing hum because they break over in blips of current twice per AC cycle. Full wave rectifiers can buzz as noted above. The CE Distribution power adapter might be having some rectifier buzz, as they do take in the AC power line and immediately rectify it to a few hundred volts of DC. Or it might be that that particular unit has a problem with its internals that's letting some buzz out. It could be that the ferrite filter and the capacitance of the power output cord happens to tune the cord into an antenna that transmits the buzzes from rectification on some particular RF frequency that your setup picks up, or that the splitter itself picks up. The guys in the RF susceptibility lab back in my previous life had stories of almost unbelievable coincidences that caused RF problems.

This is kind of an apology - I don't see a smoking gun, only places to dig further.

[Unlikekurt]

As always.  Thanks RG.  a lot to read.  but a lot to digest and think about.
Im going to have to do some sleuthing around.  I think the first place to start will be trying some other supplies.

Best

James